This invention relates generally to lasers, and, more particularly, to a waveguide-type TEA laser which incorporates therein a radioactive source for preionization of the gases contained within the laser cavity.
Lasers find use in a wide diversity of activities ranging from communication over great distances to the drilling of very accurate holes in objects.
Most lasers consist of a column of active material having a partly reflecting mirror at one end and a fully reflecting mirror at the other. The laser is primed by pumping the atoms of the active material, by means of a flash of intense light, to an excited state. With a preponderance of atoms in that state the system can be stimulated to produce a cascade of photons, all the same wavelength and all in step, by triggering the emission of energy that drops the atoms from the excited state to a lower energy state. A photon carrying this quantum of energy, on striking an excited atom, causes it to emit a photon at the same frequency, and the light wave thus released falls in step with the triggering one. Waves that travel to the sides of the column leave the system, but those that go to the ends of the column along its axis are reflected back and forth by the mirrors. The column, whose length is a whole number of wavelengths at the selected frequency, acts as a cavity resonator, and a beam of monochromatic, coherent light rapidly builds in intensity as one atom after another is stimulated to emit photons with the same energy and direction. After the laser light has built up in this way it emerges through the partly reflecting mirror at one end of the column as an intense highly directional beam.
The active medium of the conventional CO.sub.2 laser is an electrically excited mixture of carbon dioxide, nitrogen and helium. Uniform excitation of the gas mixture at atmospheric pressure, however, is not readily achieved. As the pressure is increased in the conventional low-pressure glow discharge, the characteristics of the discharge change, and at about 200 torr the glow constricts to an arc.
In some instances, a glow discharge can be maintained in the gas by making the discharge time short compared to the arc formation time or by limiting the discharge current density below that required for the formation of a constricted arc. It has been found that for a l-m discharge length at atmospheric pressure, voltages in the neighborhood of 10.sup.6 volts are required for proper excitation of CO.sub.2 lasers. To meet the requirement of a short discharge time and to lessen the requirement for such extremely high applied voltages, scientists used pulsed transverse excitation, that is, a discharge that is transverse rather than parallel to the optic axis. These transverse excited atmospheric pressure lasers are called TEA lasers.
Various methods of preionization are used in TEA-type lasers to obtain larger volumes of gas discharge and thus more energy. Preionization refers to the presence of charged particles in the gas volume prior to initiation of the discharge. These charges aid in the initiation of a large volume glow discharge of high spatial uniformity. A high-pressure pulsed molecular laser based on a discharge scheme involving preionization of the discharge volume with the aid of an auxiliary discharge from a third electrode, has been designated a double-discharge TEA laser.
One such double-discharge TEA CO.sub.2 laser system uses volume photoionization of the gases by ultraviolet radiation emitted from multiple spark discharges. In this approach, energy is supplied to the CO.sub.2 --N.sub.2 --He gas mixture by a discharge occurring between a solid cathode and a mesh anode.
The problem with the past methods of preionization by ultraviolet photoionization and/or preliminary arc discharges are many. For example,
a. these systems incorporate therein complex electrical apparatus and timing circuits; PA1 b. they produce a non-uniform preionization which yields a non-uniform excitation; PA1 c. "streamers" and/or arcs may form in the laser cavity; PA1 d. rapid degeneration of the electrodes takes place; PA1 e. hot spots may form in the laser along with distortion of the laser cavity; and PA1 f. the lasing acting may not take place at all.